As each radar was brought up it included a Mark X - IFF capability and the interrogation replies were multiplex with the radar data.
Cape Cod System fully operational by September 1953, trials began in October 1953 and concluded 15 Jun 1954 with highly favorable results.
"The Experimental SAGE Subsector (ESS) was essentially an expansion of the Cape Cod System. But, unlike the Cape Cod System, ESS was a fully operational prototype."